After two steel alloy parts have been welded together, it is usual to proceed with heat treatment known as stress-relieving treatment.
This treatment has two aims:
to eliminate or at least reduce the internal stresses of thermal origin which appear during the cooling and consequent solidification of the welded joint, and PA1 to temper the solidified metal and the adjacent zone of base metal which has also been affected by temperature, so as to improve the very mediocre ductility of these zones while the weld is in the unfinished condition.
This heat treatment generally consists simply of annealing at a temperature just below the first critical heating transformation temperature, known as AC.sub.1. Depending on the nature of the steel constituting the welded joint, this temperature lies somewhere in the range between 550.degree. C. and 750.degree. C. It is carefully chosen so as to be sufficiently high as to achieve the effects of relieving stress and of tempering, whilst at the same time being limited so as not to exceed that temperature AC.sub.1 at which the heating transformation begins nor to reach the temperature of metallurgical annealing previously experienced by the parts to be welded together, so as not to affect the properties of the base metal.
Thus, for each grade of steel, there exists an optimum temperature for heat treatment after welding.
When two steel parts of different grades are to be welded together, the choice of treatment temperature is a difficult problem. It is therefore known to use, as the treatment temperature, a temperature which is intermediate between the optimum temperatures of the two steels and which is therefore too high for the steel of lower optimum treatment temperature or, more usually, to use the temperature which corresponds to that one of the two steels which has the lower optimum treatment temperature, in which case the temperature is therefore too low for the other steel.